1. Field of the Invention
The present invention relates to a catalyst for purifying the 3 major components (i.e., hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x)), which are included in exhaust gases emitted from internal combustion engines for automobiles or the like, and a process for producing the same. More particularly, it relates to a catalyst, which can efficiently purify the 3 major components even under a fluctuating atmosphere condition, and which exhibits good heat resistance, and a process for producing the same. Note that the term "fluctuating atmosphere condition" herein means that oxidizing atmosphere and reducing atmosphere arise alternately in automotive exhaust gases.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and simultaneously reduce NO.sub.x to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant support formed of cordierite, a porous catalyst carrier layer formed of gamma-alumina and disposed on the support, and a noble metal element selected from the group consisting of platinum, palladium and rhodium and loaded on the porous catalyst carrier layer.
Japanese Examined Patent Publication (KOKOKU) No. 58- 20,307 discloses a novel 3-way catalyst whose catalytic performance is enhanced by co-employing cerium oxide (or ceria) having an oxygen storage capability as a promoter.
When loading cerium oxide working as a promoter on a porous support, an impregnation loading process and a powder mixture loading process are available. In an impregnation loading process, a monolithic support having an alumina coating layer is adsorbed by using an aqueous solution of cerium compound (e.g., cerium nitrate), and is calcinated thereafter. In a powder mixture loading process, which is an improvement of the impregnation process, a cerium oxide powder and a porous support powder, such as an alumina powder, are mixed to load cerium oxide on alumina as set forth in Japanese Unexamined Patent Publication (KOKAI) No. 60-110,334.
Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 6-279,027 discloses another mixture loading process, in which a cerium oxide sol and a zirconium oxide sol are mixed to prepare a catalyst or a catalyst support as set forth in "Supplemental Comparative Example" below. Note that this publication claims the Paris Convention Priority based on a French Application No. FR9301450, which was filed on Feb. 10, 1993.
In accordance with the impregnation loading process, the cerium oxide is turned into extremely fine particles, and can be loaded in a highly dispersed manner. The resulting automotive catalyst accordingly seems to effect the oxygen storage capability of cerium oxide maximally. In fact, however, the specific surface area of the catalyst decreases, because major part of the cerium oxide is loaded in the pores of the porous support to close the pores. Further, the cerium oxide contacts with exhaust gases at reduced probability to ineffectively effect its oxygen storage capability, because the cerium oxide is adsorbed and loaded deep in the pores. Furthermore, the specific surface area of the catalyst degrades further, because the highly dispersed cerium oxide contacts with the porous support over an increased area. Thus, the cerium oxide is likely to react with the porous support to form cerium oxide-aluminate (CeAlO.sub.3) if the porous support is formed of alumina.
In accordance with the powder mixture loading process, the resulting catalyst cannot satisfactorily utilize the oxygen storage capability of the cerium oxide; namely, it cannot keep the exhaust gases in predetermined atmosphere by storing oxygen in and releasing oxygen from the cerium oxide, because cerium oxide powder has such a large average particle diameter of 500 nm or more that the cerium oxide has a small specific surface area for contacting with the exhaust gases.
Thus, when the cerium oxide is loaded by either the impregnation loading process or the powder mixture loading process, the cerium oxide cannot fully effect its oxygen storage capability. Accordingly, the resulting catalyst cannot satisfactorily produce activities as 3-way catalyst.